1. Field of the Invention
The present invention relates to a catalyst carrier of cordierite honeycomb structure, particularly to a catalyst carrier of high strength and low thermal expansion honeycomb structure used for catalyst carriers for purifying exhaust gases from automobiles, and a method of producing the carrier.
2. Related Art Statement
Recently, thermal shock resistant and porous cordierite honeycomb ceramics have been investigated, especially as materials for catalyst carriers made of honeycomb structures used in devices for purifying hydrocarbons, carbon monoxide and nitrogen oxides in various exhaust gases. Among various devices for purifying exhaust gases, devices for purifying exhaust gases from automobiles are prevailingly used at the present time. Catalyst carriers used in such purifying devices for exhaust gases from automobile engines should possess a few important characteristic properties. One of such properties is a so-called "thermal shock resistance" which is a property of a material to withstand a crack or breakage caused by a thermal stress exerted by temperature differences. The temperature differences are formed in a honeycomb structure by temperature changes due to rapid heat generation by catalytic oxidation reactions of unburnt hydrocarbons, carbon monoxide, or the like in an exhaust gas. The thermal shock resistance is expressed by a temperature difference of the honeycomb structure of withstanding a rapid heating and a rapid cooling, and the temperature difference is known to be inversely proportional to a coefficient to thermal expansion among various properties of the honeycomb structure, and thermal shock resistance becomes larger when the coefficient of thermal expansion (to be referred to as "CTE" hereinafter) becomes smaller.
Another property that catalyst carriers made of a ceramic honeycomb structure should possess is an adhering property of catalyst active substances and catalyst substances to the catalyst carriers.
An additional important property requested to catalyst carriers made of ceramic honeycomb structure is initial activity (the right-off property) of the catalyst at the incipient period of using the catalyst made of honeycomb structure.
Heretofore, low expansion of cordierite ceramics has been publicly known. For instance, U.S. Pat. No. 3,885,977 specification discloses an oriented cordierite ceramics of CTE of less than 11.times.10.sup.-7 (1/.degree.C.) in at least one direction in a temperature range of 25.degree.-1,000.degree. C., and describes a planar orientation determinated into platelets during processing of stacked clay, which can be platelet clay such as kaoline, or the like as a cause of such orientation.
In addition, U.S. Pat. No. 4,280,845 discloses use of a magnesia source material such as talc, etc. of a limited coarse particles range of 10-50 .mu.m to obtain extremely low thermal expansion cordierite ceramics.
Meanwhile, according to recent extensive improvement in the technique of applying catalysts on carriers of honeycomb structure, a desire for highly porous catalyst carriers of honeycomb structure has been decreased. On the other hand, reduction of volume of the catalyst carriers, improvement of the catalyst property, improvement of right-off property and reduction of fuel consumption rate; reduction of pressure loss for increasing engine power; improvement of strength for reducing cost of canning the honeycomb structure in a casing; and improvements of strength and thermal shock resistance from the necessity of arranging the catalyst-equipped devices for purifying exhaust gases near the automobile engines as near as possible for enhancing the catalyst activity; have been earnestly desired. For solving these recent desires of the catalyst carriers of honeycomb structure, thinning of the partition walls of the cells of the honeycomb structure and increase of cell number per unit crosssectional area of the honeycomb structure for improving the catalyst properties, or thinning of the partition walls and decrease of cells number per unit area for decreasing pressure loss, have been studied. However, the thinning of the partition walls and the increase of cell number lead to a drawback of decreasing the strength of the porous cordierite ceramics of honeycomb structure, and a drawback of largely increasing the CTE of the cordierite honeycomb structure, because of a necessity of using far finer particles as raw materials, particularly magnesia source material, caused by decrease of width of slits of an extruding die which extrudes the raw materials into a honeycomb structure.
Densification of cordierite ceramics is rather difficult. Especially, for producing low thermal expansion cordierite ceramics of CTE of 2.0.times.10.sup.-6 /.degree.C. or less in a temperature range of from room temperature to 800.degree. C., batch composition of raw materials have to be used wherein an amount of impurities such as alkali (calcia, potassium, sodium) and the like which function as fluxes in firing the batch is limited to a very minor amount, so that cordierite honeycomb structures obtained by firing the batch composition have very low amounts of glass phases and hence are very porous and not dense. Particularly, cordierite honeycomb structures recently used as carriers of catalysts for purifying exhaust gases from automobiles necessitate CTE not greater than 1.5.times.10.sup.-6 /.degree.C. in a temperature range of room temperature to 800.degree. C., so that raw materials such as talc, kaoline, alumina, etc. of low impurities are predominantly used, and hence the fired cordierite honeycomb bodies have open porosities of about 25-45%, even if production criteria, types and particle sizes of the raw materials are selected cautiously. Especially, for producing honeycomb structures having porosities of not greater than 30%, amounts of the impurities and minimization of particle sizes of the raw materials are unavoidable, so that product honeycomb structures of CTE of 1.0.times.10.sup.-6 /.degree.C. or less in a temperature range of from room temperature to 800.degree. C. have not been obtained.
Moreover, when producing cordierite honeycomb structures of relatively small porosity, molded honeycomb structures have such a large shrinkage in the drying step and the firing step of the production process that they are liable to suffer from cracks, and honeycomb structures of large sizes have been difficult to produce at high production yields.
From the technical reasons as described above, catalyst carriers of very low thermal expansion cordierite honeycomb structures and low porosity and still of sufficiently high strength properties, have been earnestly desired.